Fingerprint recogntion sensor and terminal device

ABSTRACT

Disclosed is a fingerprint identification sensor, which comprises: a sensor unit, comprising a capacitor array formed by a plurality of capacitor induction units and having an output, a power supply and a sensor ground output; a conversion circuit, connected to the device ground of the terminal device, and the output and the sensor ground of the sensor unit, and configured to, upon modulating the driving signal to the modulated signal, output the modulated signal to the sensor ground; an energy storage capacitor, connected between the power supply and the sensor ground, and configured to stabilize an operating voltage of the sensor unit; a high-speed transistor switch, connected to the power supply, and configured to perform synchronous switch-on and switch-off according to status of the conversion circuit power supply; and a power supply, connected to the conversion circuit and the power supply of the sensor unit via the high-speed transistor switch.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Patent ApplicationNo. PCT/CN2015/072430, with an international filing date of Feb. 6,2015, designating the United States, now pending, which is based onChinese Patent Application No. 201410849191.9, filed Dec. 30, 2014. Thecontents of these specifications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to the technical field of fingerprintidentification, and in particular, relates to a fingerprintidentification sensor and a terminal device.

Description of the Related Art

The fingerprint identification technology is widely applied in the fieldof the electronic security, and is a reliable method for implementingidentity authentication. A capacitive fingerprint identification sensoris one of the widely used fingerprint sensors, and is formed by aminiaturized capacitor polar plate array. An upper surface of the polarplate is covered by an insulation plate, and when a user puts his or herfinger on the insulation plate, the skin forms the other plate of acapacitor array. Since the distances between ridges and valleys offingerprints in different zones and the capacitance plate array aredifferent, such that capacitance of each capacitor polar plate variesaccordingly. In this way, a fingerprint image is acquired.

FIG. 1 illustrates a common capacitive fingerprint identificationsensor, which comprises a sensor unit, a driving amplifier 101, adriving metal ring 103 and a power supply (not illustrated in FIG. 1)supplying power for the sensor unit. The sensor unit comprises acapacitor array formed by a plurality of capacitor induction units 102,and FIG. 1 schematically illustrates any one of the plurality ofcapacitor induction units 102. The sensor unit outputs a driving signalto the driving amplifier 101, and the driving amplifier 101 amplifiesthe driving signal and outputs the amplified driving signal to thedriving metal ring 103. When a finger 104 presses the capacitor array ofthe sensor unit, a coupling capacitance between the finger and thecapacitor induction units 102 is C_(X). The driving signal is coupled tothe finger through a capacitance C_(T) from the driving metal ring 103,the capacitance C_(X) between ridges and valleys of fingerprints indifferent zones and the capacitor induction units 102 varies, and thecapacitor induction units 102 detects that the voltage variesaccordingly. In this way, the fingerprint image is acquired.

The fingerprint identification sensor having this structure needs anexternal metal ring 103. However, to enlarge capacitance C_(T) as largeas possible, and reduce signal attenuation, the region where thefingerprint identification sensor is located needs to be hollowed out toplace the metal ring 103, such that the finger may directly touch themetal ring 103. However, in some application scenarios, for example, theappearance design of a mobile phone and a tablet personal computer, toachieve mirror-like screen effect and improve water proof performance,it is not suitable to open a hole on the screen, which restricts theapplication scope of the fingerprint identification sensor.

In addition, an equivalent capacitance formed by series connection of acapacitance C_(S) and a capacitance C_(M), in which C_(S) is formedbetween a terminal device equipped with the fingerprint identificationsensor and the ground, and C_(M) is formed between the human body andthe ground, and a capacitance C_(H), formed directly between the humanbody and the terminal device will cause attenuation to the drivingsignal on the driving metal ring 103. When the terminal device isprovided with a metal housing, and the device is held in one user'shand, the attenuation to the driving signal will be more serious,thereby reducing clarity of the fingerprint image, and affecting thefingerprint identification effect.

SUMMARY OF THE INVENTION

Embodiments of the present invention are intended to provide afingerprint identification sensor and a terminal device, to broaden theapplication scope of the fingerprint identification sensor and improvethe fingerprint identification effect.

To this end, an embodiment of the present invention provides afingerprint identification sensor, applicable to a terminal device,wherein the fingerprint identification sensor comprises:

a sensor unit, comprising a capacitor array formed by a plurality ofcapacitor induction units, and having an output, a power supply and asensor ground, the output outputting a driving signal; and

a modulation circuit, connected to a device ground of the terminaldevice, the output, the power supply and the sensor ground of the sensorunit, and configured to, upon modulating the driving signal to amodulated signal, output the modulated signal to the sensor ground,wherein a voltage of the power supply varies with the modulated signal.

Preferably, the modulation circuit comprises:

a conversion circuit, connected to the device ground of the terminaldevice, the output and the sensor ground of the sensor unit, andconfigured to, upon modulating the driving signal to the modulatedsignal, output the modulated signal to the sensor ground;

an energy storage capacitor, connected between the power supply and thesensor ground, and configured to stabilize an operating voltage of thesensor unit;

a high-speed transistor switch, connected to the power supply, andconfigured to perform synchronous switch-on and switch-off according tostatus of the conversion circuit, such that a voltage of the powersupply varies with the modulated signal; and

a power source, connected to the conversion circuit and the power supplyof the sensor unit via the high-speed transistor switch to supply powerfor the conversion circuit and the sensor unit.

Preferably, the sensor unit is connected to a main control module of theterminal device via a communication port, and configured to output adriving signal having a low level between modulation idle intervals,such that a level of the device ground is approximately equal to a levelof the sensor ground.

Preferably, the sensor unit is directly connected to the main controlmodule via a conducting wire, and when the conversion circuit modulatesthe driving signal, the communication port is maintained at a low level.Or, the fingerprint identification sensor further comprises a resistorarray, the sensor unit is connected to the main control module via theresistor array. Or the fingerprint identification sensor furthercomprises a relay module, the sensor unit is connected to the maincontrol module of the terminal device via the relay module.

Preferably, when the sensor unit is connected to the main control modulevia the relay module, the relay module receives data sent by the sensorunit and caches the data, and the main control module acquires the datafrom the relay module.

Preferably, the sensor unit and the relay module are integrated in asensor chip.

Preferably, the power source is connected to the sensor unit via a powerswitch, ON or OFF of the power switch being controlled by the maincontrol module or the relay module.

Preferably, the conversion circuit, the high-speed transistor switch,the power switch, and the relay module are integrated in a chip.

Preferably, the conversion circuit is formed by any one of or anycombination of at least two of a transistor, an operational amplifier, aphase inverter and a digital buffer gate, plus a resistor or/and acapacitor.

Preferably, the conversion circuit is formed by two phase inverters andone resistor, the phase inverters comprising a first phase inverter anda second phase inverter; wherein a positive input power supply of thefirst phase inverter is connected to the power supply of the sensorunit, a negative input power supply of the first phase inverter isconnected to the device ground, an input of the first phase inverter isconnected to the output of the sensor unit and connected to a negativeinput power supply of the second phase inverter and the device groundvia the resistor, and an output of the first phase inverter is connectedto an input of the second phase inverter; and an positive input powersupply of the second phase inverter is connected to the power source, anegative input power supply of the second phase inverter is connected tothe device ground, and an output of the second phase inverter isconnected to the sensor ground.

Preferably, the first inverter is formed by a first NMOS transistor, afirst PMOS transistor and a first resistor; wherein a gate of the firstNMOS transistor is connected to a gate of the first PMOS transistor toform the input of the first phase inverter. A source of the first PMOStransistor is used as the positive input power supply of the first phaseinverter, and a source of the first NMOS transistor is used as thenegative input power supply of the first inverter. A drain of the firstNMOS transistor is connected to a drain of the first PMOS transistor viathe first resistor, and both drains of the first NMOS transistor and thefirst PMOS transistor can be used as an output of the first phaseinverter. The second phase inverter is formed by a second NMOStransistor, a second PMOS transistor and a second resistor, connectionsthereof being the same as those of the first phase inverter.

Preferably, the high-speed transistor switch is formed by any one of orany combination of at least two of a Schottky diode, a fast recoverydiode, a crystal triode, a field effect transistor and a siliconcontrolled rectifier.

Preferably, the fingerprint identification sensor further comprises alow dropout regulator, wherein the low dropout regulator is connectedbetween the power supply and the energy storage capacitor.

An embodiment of the present invention further provides a terminaldevice. The terminal device comprises a fingerprint identificationsensor, wherein the fingerprint identification sensor comprises:

a sensor unit, comprising a capacitor array formed by a plurality ofcapacitor induction units, and having an output, a power supply and asensor ground, the output outputting a driving signal; and

a conversion circuit, connected to the device ground of the terminaldevice, the output and the sensor ground of the sensor unit, andconfigured to, upon modulating the driving signal to the modulatedsignal, output the modulated signal to the sensor ground;

an energy storage capacitor, connected between the power supply and thesensor ground, and configured to stabilize an operating voltage of thesensor unit;

a high-speed transistor switch, connected to the power supply, andconfigured to perform synchronous switch-on and switch-off according tostatus of the conversion circuit, such that a voltage of the powersupply varies with the modulated signal; and

a power source, connected to the conversion circuit and the power supplyof the sensor unit via the high-speed transistor switch to supply powerfor the conversion circuit and the sensor unit.

In the fingerprint identification sensor according to the embodiments ofthe present invention, a high-speed transistor and an energy storagecapacitor form a power supply circuit of a sensor unit, and a conversioncircuit modulates a driving signal output by the sensor unit and thendrives a sensor ground of the sensor unit. Since the driving signal ofthe sensor unit is modulated to a modulated signal, when a fingerpresses a capacitor induction unit of the sensor unit, a loop is formedfor the modulated signal based on a capacitance C_(X) between thedriving signal and the finger and a capacitance between the human bodyand a device ground of a terminal device. When C_(X) varies, ameasurement voltage of the capacitor induction unit of the sensor unitvaries accordingly, such that a fingerprint image may be acquired andfingerprint identification is implemented.

Since the fingerprint identification sensor according to the embodimentsof the present invention does not need a metal ring, it is unnecessaryto form a hole in the surface of a terminal device to place the drivingmetal ring. Therefore, the appearance design of the terminal device willnot be affected, and the fingerprint identification sensor can beapplied to terminal devices such as a mobile phone and a tablet personalcomputer not expected to be holed. Accordingly, the application scope ofthe fingerprint identification sensor is broadened.

In the meantime, an equivalent capacitance formed by series connectionof a capacitance C_(S) of the terminal device to the ground and acapacitance C_(M) of the human body to the ground, and a capacitanceC_(H) of the human body directly to the terminal device would cause noattenuation impact onto the driving signal. On the contrary, the greaterthese capacitances are, the stronger the coupling is, the higher thevoltage at two terminals of C_(X), and the clearer the fingerprintimage, which, therefore, improves the fingerprint identification effect.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic structural diagram of a fingerprint identificationsensor in the related art;

FIG. 2 is a schematic circuit structural diagram of a fingerprintidentification sensor according to an embodiment of the presentinvention;

FIG. 3 is a schematic diagram illustrating circuit connection of aconversion circuit according to an embodiment of the present invention;

FIG. 4 is a schematic diagram illustrating circuit connection of thefingerprint identification sensor according to a first embodiment of thepresent invention;

FIG. 5 is an operating time sequence diagram of the conversion circuitaccording to an embodiment of the present invention;

FIG. 6 is a schematic diagram illustrating circuit connection of thefingerprint identification sensor according to a second embodiment ofthe present invention;

FIG. 7 is a schematic diagram illustrating circuit connection of thefingerprint identification sensor according to a third embodiment of thepresent invention;

FIG. 8 is a schematic diagram illustrating circuit connection of thefingerprint identification sensor according to a fourth embodiment ofthe present invention; and

FIG. 9 is a schematic diagram illustrating application of thefingerprint identification sensor to a mobile phone according to anembodiment of the present invention.

The attainment of the objectives, functional features and advantages ofthe present invention are further described hereinafter with referenceto the specific embodiments and the accompanying drawings.

DETAILED DESCRIPTION OF THE EMBODIMENTS

It should be understood that the embodiments described here are onlyexemplary ones for illustrating the present invention, and are notintended to limit the present invention.

The fingerprint identification sensor according to the present inventionis a capacitive fingerprint identification sensor, which may beapplicable to a mobile terminal, a tablet personal computer, a palmdevice, a smart wearable device, a multimedia player, a laptop computer,a desktop computer, and an access control security system and the liketerminal device. The fingerprint identification sensor comprises asensor unit and a modulation circuit. The sensor unit comprises acapacitor array formed by a plurality of capacitor induction units, andhas an output, a power supply and a sensor ground. The output outputs adriving signal. The modulation circuit is connected to a device groundof a terminal device, and the output, the power supply and the sensorground of the sensor unit; and configured to, upon modulating thedriving signal to a modulated signal, output the modulated signal to thesensor ground, wherein a voltage of the power supply varies with themodulated signal.

In an embodiment of the present invention, the fingerprintidentification sensor is as illustrated in FIG. 2, and comprises asensor unit 210 and a modulation circuit. The modulation circuitcomprises: a conversion circuit 230, an energy storage capacitor 240, ahigh-speed transistor switch 250 and a power source 260. The sensor unit210 has an output, a power supply (sensor VDD, hereinafter referred toas SVDD) and a sensor ground (hereinafter referred to as SGND). Theoutput is connected to the conversion circuit 230. The conversioncircuit 230 is connected to a device ground (hereinafter referred to asGND) of a terminal device and the SGND of the sensor unit. Thehigh-speed transistor switch 250 is connected to the SVDD of the sensorunit 210, and the high-speed transistor switch 250 may be formed by anyone or any combination of at least two of a Schottky diode, a fastrecovery diode, a crystal triode, a field effect transistor and asilicon controlled rectifier, wherein the practice includes singleformation, formation by a plurality of such components by means ofseries connection or/and parallel connection, and formation by aplurality of such components by means of multiple types of seriesconnections or/and parallel connections. The power source 260 isconnected to the conversion circuit 230, and is connected to the SVDD ofthe sensor unit 210 via the high-speed transistor switch 250, whichsupplies power for the conversion circuit 230 and the sensor unit 210.The energy storage capacitor 240 is connected between the SVDD and theSGND of the sensor unit 210, to stabilize the operating voltage of thesensor unit 210. The high-speed transistor switch 250 and the energystorage capacitor 240 form a power supply circuit of the sensor unit210.

The conversion circuit 230 may be formed by any one or any combinationof at least two of a transistor, an operational amplifier, a phaseinverter, a level shifter and a digital buffer gate, plus a resistoror/and a capacitor. The conversion circuit 230 is preferably asillustrated in FIG. 3, and is formed by two phase inverters (231, 232)and a resistor R3. The two phase inverters comprise a first phaseinverter 231 and a second phase inverter 232. A positive input powersupply of the first phase inverter 231 is connected to the SVDD of thesensor unit 210, a negative input power supply of the first phaseinverter 231 is connected to the GND of the terminal device, an input ofthe first phase inverter 231 is connected to the output of the sensorunit 210 and connected to a negative input power supply of the secondphase inverter 232 and the GND of the terminal device via a resistor R3,and an output of the first phase inverter 231 is connected to an inputof the second phase inverter 232; and an positive input power supply ofthe second phase inverter 232 is connected to the power source 260, anegative input power supply of the second phase inverter 232 isconnected to the GND of the terminal device, and an output of the secondphase inverter 232 is connected to the SGND of the sensor unit.

The phase inverter is preferably formed by two transistors and aresistor, wherein the transistor is a metal oxide semiconductor fieldeffect transistor (hereinafter referred to as MOSFET or MOS transistor),including a positive channel metal oxide semiconductor field effecttransistor (PMOSFET or PMOS transistor) and a negative channel metaloxide semiconductor field effect transistor (NMOSFET or NMOStransistor). As illustrated in FIG. 3, gates of PMOS transistors (Q1,Q3) are connected to gates of NMOS transistors (Q2, Q4) to form input ofthe phase inverters (231, 232), sources of the PMOS transistors (Q1, Q3)act as positive input power supply of the phase inverters (231, 232),sources of the NMOS transistors (Q2, Q4) act as negative input powersupply of the phase inverters (231, 232), drains of the PMOS transistors(Q1, Q3) are connected to drains of the NMOS transistors (Q2, Q4) viaresistors (R1, R2), and both the drains of the PMOS transistors (Q1, Q3)and the drains of NMOS transistors (Q2, Q4) may act as the output of thephase inverters (231, 232).

The sensor unit 210 comprises a driving amplifier 220, and a capacitorarray formed of a plurality of capacitor induction units 211. FIG. 2schematically illustrates any one of the plurality of capacitorinduction units 211. An output of the driving amplifier 220, acting asthe output of the sensor unit, is connected to the conversion circuit230. The driving amplifier 220 amplifies a driving signal of the sensorunit 210 and then outputs the amplified driving signal to the conversioncircuit 230. The conversion circuit 230 modulates the driving signal toa modulated signal and then outputs the modulated signal to the SGND ofthe sensor unit 210. The high-speed transistor switch 250 performssynchronous switch-on or switch off according to status of theconversion circuit 230, such that the voltage of the SVDD of the sensorunit 210 varies with the modulated signal.

The equivalent capacitance formed by series connection of thecapacitance C_(S) of the terminal device to the ground and thecapacitance C_(M) of the human body to the ground, and the capacitanceC_(H) of the human body directly to the terminal device would causecoupling between the human body and the GND of the terminal device. Suchcoupling is present in any application scenario. Since the drivingsignal of the sensor unit 210 is modulated to a modulated signal, when afinger presses a capacitor induction unit 211 of the sensor unit 210, aloop is formed for the modulated signal based on the capacitance C_(X)between the modulated signal and the finger, and the capacitance betweenthe human body and the GND of the terminal device. When C_(X) varies, ameasurement voltage of the capacitor induction unit 211 of the sensorunit 210 varies accordingly, such that a fingerprint image may beacquired.

FIG. 4 illustrates a first embodiment of specific application of thefingerprint identification sensor according to the present invention. Inthis embodiment, the sensor unit is integrated in a sensor chip, whereinthe sensor chip comprises a scanning module and a serial peripheralinterface (SPI) module. The scanning module outputs a driving signal toscan the capacitor array. The SPI module provides an SPI interface whichacts as a communication interface of the sensor chip which is connectedto a communication interface of a main control module of the terminaldevice, in order to communicate with the main control module via thecommunication interface of the main control module. For example, thesensor chip sends fingerprint image data to the main control module, andthe main control module sends a control command or the like to thesensor chip. The communication interface may be an inter-integratedcircuit (I2C) interface, a serial/parallel interface or the like, inaddition to the SPI interface.

The conversion circuit 230, as illustrated in FIG. 3, is formed by fourtransistors and three resistors. The transistors may be MOS transistors,including PMOS transistors Q1 and Q3, and NMOS transistors Q2 and Q4;and the resistors include resistors R1, R2 and R3.

With reference to FIG. 4 and FIG. 5, the working principles of theconversion circuit are as follows:

The sensor chip scans the capacitor array and reads voltages ofdifferent capacitor induction units, and the scanning manner iscontrolled by scanning module. The scanning module outputs a drivingsignal TX, wherein the driving signal TX is a high-frequency alternatingcurrent signal, which may be specifically a sine wave, a square wave, atriangular wave or the like. In this embodiment, the driving signal TXis a wave square signal having a frequency of 800 kHz, which may alsohave another frequency value. The driving signal TX is modulated to theSGND via the conversion circuit 230. The peak value of the modulatedsignal is determined by the voltage supplied by the power source 260. Inthis embodiment, the peak value of the modulated signal is approximateto the input voltage 2.8 V, which may also be another voltage value.

The sensor chip outputs the driving signal TX having a low level SGNDbetween modulation idle intervals, and a voltage difference between thedriving signal TX and the SVDD is approximate to 2.8 V. The PMOStransistor Q1 is conducted, the resistance of R1 is far greater than theconductive internal resistance of the PMOS transistor Q1 and the NMOStransistor Q2. Due to existence of the resistor R1, no matter the NMOStransistor Q2 is conducted or cut off, the voltage of the node TX_S ispulled up to the SVDD, and then the SVDD voltage of the TX_S causes theNMOS transistor Q4 to be conducted and the PMOS transistor Q3 to be cutoff. In this case, the NMOS transistor Q4 connects the SGND to the GND,and then causes the driving signal TX to be maintained at the GNDvoltage. In this way, the NMOS transistor Q2 is cut off, and the PMOStransistor Q1, the PMOS transistor Q3 and the NMOS transistor Q4 remainin unchanged status, and the voltage of the SGND is maintained at theGND, as illustrated by stage 1 in the waveforms in FIG. 5.

When the driving signal TX changes to a high level, the driving signalTX is approximately equal to the SVDD voltage. In this case, the PMOStransistor Q1 is cut off, the NMOS transistor Q2 is conducted, and thevoltage of the node TX_S is pulled down to the GND; then the low voltageof the TX_S causes the PMOS transistor Q3 to be conducted, the NMOStransistor Q4 to be cut off, and in this case, the PMOS transistor Q3forcibly pulls the SGND up to 2.8 V. Since the voltages applied at twoterminals of the energy storage capacitor 240 may not be abruptlychanged and are approximately maintained at 2.8 V, the SVDD voltage maybe forcibly pumped to about 5.6 V, and the high-speed transistor switch250 is automatically switched off due to reverse bias. The SVDD voltagechanges to 5.6 V, and the voltage of the driving signal is approximatelyequal to the voltage of the SVDD. Therefore, the voltage of the drivingsignal TX is approximately 5.6 V, the MOS transistors Q1 to Q4 remain inunchanged status, and the SGND voltage remains stable at 2.8 V, asillustrated in stage 2 in the waveforms in FIG. 5.

When the driving signal TX changes from a high level to a low level, thevoltage of the driving signal TX is approximately equal to the voltageof the SGND, about 2.8 V. In this case, the SVDD voltage is 5.6 V, thePMOS transistor Q1 and the NMOS transistor Q2 are concurrentlyconducted. Since the voltage of the resistor R1 and TX_S isapproximately equal to the SVDD, the NMOS transistor Q4 is conducted,the PMOS transistor Q4 is cut off, and the SGND is connected to the GND.Afterwards, the process in stage 1 is repeated. The process that thedriving signal TX changes from a high level to a low level is asillustrated in stage 3 in the waveforms in FIG. 5.

Described above is the operating process of the conversion circuit. TheSGND of the sensor chip may be modulated to a square waveform which hasthe same frequency and phase as the driving signal TX, and the voltageof the modulated signal is equal to a supply voltage supplied by thepower source 260.

After the sensor chip acquires fingerprint image data, the data needs tobe transmitted to the main control module of the terminal device, andthe main control module processes the data and identifies a fingerprintobject. In this embodiment, an SPI interface is connected to thefingerprint sensor chip and the main control module, the communicationinterface may be practiced in a plurality of manners, which is notlimited to the SPI interface according to this embodiment.

When the conversion circuit 230 modulates the SGND to an alternatingcurrent square wave signal, the signal of the SPI interface also aremodulated to modulated signal. The modulated signal of the SPI interfacemay not be directly identified by an SPI module of the main controlmodule, thereby resulting in communication exception. However, thesensor chip does not uninterruptedly scan the capacitor array, anddifferent capacitor induction units may be scanned at intervals. In mostcases, the interval is sufficiently large, such that the data may betransmitted directly by using the interval. Therefore, it is onlyrequired that during the scanning interval, that is, the modulation idleinterval, the scanning module in the sensor chip outputs a low leveldriving signal, such that the level of the SGND is approximately equalto the level of the GND. In this case, the SPI module in the sensor chipis capable of normally communicating with the SPI module of the maincontrol module.

Further, the fingerprint identification sensor further comprises aresistor array 270, and the sensor chip is connected to the main controlmodule via the resistor array 270. The resistors in the resistor array270 may be series resistors, pull-up and pull-down resistors, pull-upand pull-down TVS diode or the like. When the SGND of the sensor chip ismodulated to an alternating current square wave, the voltage on thecommunication signal line may be a high voltage (in this embodiment, thevoltage is 5.6 V, which is a high voltage relative to 2.8 V). This maycause latent damages to the communication interface. The resistor array270 may avoid this problem, and the resistance of the resistor array isabout 20 to 2000 ohms, as required.

In some embodiments, the resistor array 270 may be omitted, and thesensor chip is connected to the main control module directly via aconducting wire. In this case, when the conversion circuit 230 modulatesthe driving signal (that is, when the signal of the SGND is modulated toa square wave signal), the communication interface (for example, SPIinterface in this embodiment) is maintained at a low level, and then thehighest voltage on the communication signal line is only 2.8 V, therebyavoiding the above problem.

Further, the power source 260 is connected to the sensor chip via acontrollable power switch 280, and the main control module controls ONor OFF (switch-on or switch-off) of the power switch 280 and hencecontrol the power supply to the conversion circuit 230 and the sensorchip. When the sensor chip is in a modulation idle state, the powerswitch 280 is switched off, thereby reducing the system powerconsumption. In this case, when the SGND of the sensor chip is modulatedto an alternating current square wave, the reset pin RST of the sensorchip is also modulated, resulting in an external reset exception. Undersuch circumstance, the sensor chip may be powered on again and reset bycontrolling the power switch 280. The power switch 280 may be formed bya crystal triode or/and a field effect transistor, which may be formedby a single one or a combination of multiples ones, for example, PMOStransistor(s).

Further, a low dropout regulator may be connected between the SVDD ofthe sensor chip and the energy storage capacitor, to improve stabilityof the power supplied for the sensor chip.

FIG. 6 illustrates a second embodiment of the fingerprint identificationsensor according to the present invention. This embodiment differs fromthe first embodiment in that the sensor chip is connected to the maincontrol module of the terminal device via a relay module. The relaymodule has two SPI interfaces, SPI-A and SPI-B, wherein the SPI-Ainterface is connected to the SPI interface of the sensor chip, and theSPI-B interface is connected to the SPI interface of the main controlmodule (which may also be connected via other communication interfaces).The relay module, via the SPI-A interface, receives and caches the datasent by the sensor chip, and the main control module acquires the datavia the SPI-B interface of the relay module. For example, the relaymodule sends the data, via the SPI-B interface, to the main controlmodule according to a command issued by the main control module.Meanwhile, the relay module further receives a command from the maincontrol module, and controls the sensor chip according to the receivedcommand.

In the first embodiment as illustrated in FIG. 4, the main controlmodule needs to timely receive the data of the sensor chip and processthe data within the scanning interval (that is, within the modulationidle interval) of the capacitor array. However, the scanning intervalmay be less than 1 ms. Such a high requirement on timeliness is hard tobe accommodated for most main control systems, which restricts theapplication scope. In this embodiment, the relay module is formed by amicro control unit (MCU), and the MCU has two groups of independent SPIcommunication interfaces, SPI-A and SPI-B. The MCU controls the SPI-Ainterface to receive the data of the sensor chip and cache the databetween modulation idle intervals, and to transmit the data to the maincontrol module via the SPI-B interface according to a command issued bythe main control module when the main control module is idle, whichgreatly reduces the requirement on timeliness of the main controlmodule, and improves the application scope. The MCU is furtherresponsible for forwarding a command from the main control module to thesensor chip, and controlling the square wave signal. In addition, therelay module may be also formed by any one or any combination of atleast two of an MCU, a field programmable gate array (FPGA), a flash anda first in first out (FIFO) buffer.

In the embodiment involving a relay module, the main control module orthe relay module may control ON or OFF of the power switch 280.

FIG. 7 illustrates a third embodiment of the fingerprint identificationsensor according to the present invention. This embodiment differs fromthe second embodiment in that the conversion circuit 230, the high-speedtransistor switch 250, the power switch 280 and the relay module areintegrated in the same chip, and a fingerprint identification system ismainly formed by two chips, wherein the peripheral circuits of the chipsare simpler, to accommodate the demand on a small-size terminal device.

FIG. 8 illustrates a fourth embodiment of the fingerprint identificationsensor according to the present invention. In this embodiment, thesensor unit and the relay module are integrated in a sensor chip. Inthis way, the size of a fingerprint identification system is furtherreduced, to accommodate to the demand on a small-size terminal device.

The fingerprint identification sensor according to the embodiment of thepresent invention is capable of operating without a driving metal ring.Therefore, there is no need to open a hole in the surface of theterminal device, and the sensor only needs to be installed in a specificzone under an insulation plate of the device, which implements theinvisible fingerprint sensor (IFS) technology. FIG. 9 illustrates anapplication scenario where the IFS technology is applied to a smartphone. The smart phone comprises a display screen cover plate 10,wherein a screen display region 20 is located at a central region of thescreen, and a fingerprint identification sensor 30 is “hidden” under thedisplay screen cover plate 10. A hole does not need to be opened in thedisplay screen cover plate 10 to receive the driving metal ring.Therefore, the appearance design of the display screen cover plate 10 isnot greatly affected, and a full mirror-like screen effect may beimplemented.

In addition, an equivalent capacitance formed by series connection of acapacitance C_(S) of the terminal device to the ground and a capacitanceC_(M) of the human body directly to the ground, and a capacitance C_(H)of the human body directly to the terminal device would cause noattenuation impact onto the driving signal. On the contrary, the greaterthese capacitances are, the stronger the coupling is, the higher thevoltage at two terminals of Cx, and the clearer the fingerprint image.In this way, the problem that the metal housing of the device causesattenuation to the driving signal and thus the clarity of thefingerprint signal is lowered is solved.

An embodiment of the present invention further provides a terminaldevice. The terminal device comprises a fingerprint identificationsensor. The fingerprint identification sensor comprises: a sensor unit,a conversion circuit, an energy storage capacitor, a high-speedtransistor switch and a power source. The sensor unit has an output, apower supply and a sensor ground, the output outputting a drivingsignal. The conversion circuit is connected to the device ground of theterminal device, the output and the sensor ground of the sensor unit,and is configured to, upon modulating the driving signal to themodulated signal, output the modulated signal to the sensor ground. Theenergy storage capacitor is connected between the power supply and thesensor ground, and is configured to stabilize an operating voltage ofthe sensor unit. The high-speed transistor switch is connected to thepower supply, and is configured to perform synchronous switch-on andswitch-off according to status of the conversion circuit, such that avoltage of the power supply varies with changes of the modulated signal.The power source is connected to the conversion circuit and the powersupply of the sensor unit via the high-speed transistor switch to supplypower for the conversion circuit and the sensor unit. The fingerprintidentification sensor described in this embodiment is the fingerprintidentification sensor involved in the above embodiments of the presentinvention, which is thus not described herein any further.

Since the terminal device according to the embodiment of the presentinvention employs the fingerprint identification sensor which does notneed a metal ring, it is unnecessary to form a hole in the surface of aterminal device to place the driving metal ring. Therefore, theappearance design of the terminal device will not be affected. In themeantime, an equivalent capacitance formed by series connection of acapacitance C_(S) of the terminal device to the ground and a capacitanceC_(M) of the human body directly to the ground, and a capacitance C_(H)of the human body to the terminal device would cause no attenuationimpact onto the driving signal. On the contrary, the greater thesecapacitances are, the stronger the coupling is, the higher the voltageat two terminals of C_(X), and the clearer the fingerprint image, which,therefore, improves the fingerprint identification effect.

The preferred embodiments of the present invention are described withreference to the accompanying drawings, but the scope of the presentinvention is not limited to such embodiments. A person skilled in theart would derive various modifications or variations to practice theembodiment of the present invention without departing from the scope andessence of it. For example, the features disclosed in one embodiment maybe used in another embodiment to produce a new embodiment. Anymodifications, equivalent replacements and improvements made within thetechnical concept of the present invention shall fall within the scopedefined by the claims of the present invention.

INDUSTRIAL PRACTICABILITY

In the fingerprint identification sensor according to the embodiments ofthe present invention, a high-speed transistor and an energy storagecapacitor form a power supply circuit of a sensor unit, and a conversioncircuit modulates a driving signal output by the sensor unit to drive asensor ground of the sensor unit. Since the driving signal of the sensorunit is modulated to a modulated signal, when a finger presses acapacitive sensing unit of the sensor unit, the modulated signal is in aloop formed by a human body and a device ground of a terminal accordingto a capacitance C_(X) between the driving signal and the finger. WhenC_(X) varies, a measurement voltage of the capacitive sensing unit ofthe sensor unit varies accordingly, such that a fingerprint image may beacquired and fingerprint identification is implemented.

Since the fingerprint identification sensor according to the presentinvention does not need a metal ring, it is unnecessary to form a holein the surface of a terminal device to place the driving metal ring, theappearance design of the terminal device will not be affected, and thefingerprint identification sensor can be applied to terminal devicessuch as a mobile phone and a tablet personal computer not expected to beholed; the application scope of the fingerprint identification sensor isbroadened, and the fingerprint identification effect is improved.

In the meantime, an equivalent capacitance formed by series connectionof a capacitance C_(S) of the terminal device to the ground and acapacitance C_(M) of the human body directly to the ground, and acapacitance C_(H) of the human body to the terminal device would causeno attenuation impact onto the driving signal. On the contrary, thegreater these capacitances are, the stronger the coupling is, the higherthe voltage at two terminals of C_(X), and the clearer the fingerprintimage, which, therefore, improves the fingerprint identification effect.

What is claimed is:
 1. A fingerprint identification sensor, applicableto a terminal device, comprising: a sensor unit, comprising a capacitorarray formed by a plurality of capacitor induction units, and having anoutput, a power supply and a sensor ground, the output outputting adriving signal; and a modulation circuit, connected to a device groundof the terminal device, and the output, the power supply and the sensorground of the sensor unit, and configured to, upon modulating thedriving signal to a modulated signal, output the modulated signal to thesensor ground, wherein a voltage of the power supply varies with themodulated signal.
 2. The fingerprint identification sensor according toclaim 1, wherein the modulation circuit comprises: a conversion circuit,connected to the device ground of the terminal device, and the outputand the sensor ground of the sensor unit, and configured to, uponmodulating the driving signal to the modulated signal, output themodulated signal to the sensor ground; an energy storage capacitor,connected between the power supply and the sensor ground, and configuredto stabilize an operating voltage of the sensor unit; a high-speedtransistor switch, connected to the power supply, and configured toperform synchronous switch-on and switch-off according to status of theconversion circuit, such that a voltage of the power supply varies withthe modulated signal; and a power source, connected to the conversioncircuit and the power supply of the sensor unit via the high-speedtransistor switch to supply power for the conversion circuit and thesensor unit.
 3. The fingerprint identification sensor according to claim2, wherein the sensor unit is connected to a main control module of theterminal device via a communication port, and configured to output adriving signal having a low level between modulation idle intervals,such that a level of the device ground is approximately equal to a levelof the sensor ground.
 4. The fingerprint identification sensor accordingto claim 3, wherein: the sensor unit is directly connected to the maincontrol module via a conducting wire, and the communication port ismaintained at a low level while the conversion circuit modulates thedriving signal; or the fingerprint identification sensor furthercomprises a resistor array, the sensor unit being connected to the maincontrol module via the resistor array; or the fingerprint identificationsensor further comprises a relay module, the sensor unit being connectedto the main control module of the terminal device via the relay module.5. The fingerprint identification sensor according to claim 4, whereinwhen the sensor unit is connected to the main control module via therelay module, the relay module receives data sent by the sensor unit andcaches the data, and the main control module acquires the data from therelay module.
 6. The fingerprint identification sensor according toclaim 5, wherein the sensor unit and the relay module are integrated ina sensor chip.
 7. The fingerprint identification sensor according toclaim 3, wherein the power source is connected to the sensor unit via apower switch, ON or OFF of the power switch being controlled by the maincontrol module.
 8. The fingerprint identification sensor according toclaim 5, wherein the power source is connected to the sensor unit via apower switch, ON or OFF of the power switch being controlled by the maincontrol module or the relay module.
 9. The fingerprint identificationsensor according to claim 8, wherein the conversion circuit, thehigh-speed transistor switch, the power switch, and the relay module areintegrated in a chip.
 10. The fingerprint identification sensoraccording to claim 2, wherein the conversion circuit is formed by anyone or any combination of at least two of a transistor, an operationalamplifier, a phase inverter, a level shifter and a digital buffer gate,in addition to a resistor or/and a capacitor.
 11. The fingerprintidentification sensor according to claim 10, wherein the conversioncircuit is formed by two phase inverters and one resistor, the phaseinverters comprising a first phase inverter and a second phase inverter;wherein a positive input power supply of the first phase inverter isconnected to the power supply of the sensor unit, a negative input powersupply of the first phase inverter is connected to the device ground, aninput of the first phase inverter is connected to the output of thesensor unit and connected to a negative input power supply of the secondphase inverter and the device ground via the resistor, and an output ofthe first phase inverter is connected to an input of the second phaseinverter; and an positive input power supply of the second phaseinverter is connected to the power source, a negative input power supplyof the second phase inverter is connected to the device ground, and anoutput of the second phase inverter is connected to the sensor ground.12. The fingerprint identification sensor according to claim 11, whereinthe first inverter is formed by a first NMOS transistor, a first PMOStransistor and a first resistor; wherein a gate of the first NMOStransistor is connected to a gate of the first PMOS transistor to formthe input of the first phase inverter, a source of the first PMOStransistor is used as the positive input power supply of the first phaseinverter, a source of the first NMOS transistor is used as the negativeinput power supply of the first inverter, a drain of the first NMOStransistor is connected to a drain of the first PMOS transistor via thefirst resistor, and either the drain of the first NMOS transistor or thedrain of the first PMOS transistor is used as an output of the firstphase inverter; and the second phase inverter is formed by a second NMOStransistor, a second PMOS transistor and a second resistor, connectionsthereof being the same as those of the first phase inverter.
 13. Thefingerprint identification sensor according to claim 2, wherein thehigh-speed transistor switch is formed by any one or any combination ofat least two of a Schottky diode, a fast recovery diode, a crystaltriode, a field effect transistor and a silicon controlled rectifier.14. The fingerprint identification sensor according to claim 2, furthercomprising a low dropout regulator, wherein the low dropout regulator isconnected between the power supply and the energy storage capacitor. 15.A terminal device, comprising a fingerprint identification sensor;wherein the fingerprint identification sensor comprises: a sensor unit,comprising a capacitor array formed by a plurality of capacitorinduction units, and having an output, a power supply and a sensorground, the output outputting a driving signal; and a modulationcircuit, connected to a device ground of the terminal device, and theoutput, the power supply and the sensor ground of the sensor unit, andconfigured to, upon modulating the driving signal to a modulated signal,output the modulated signal to the sensor ground, wherein a voltage ofthe power supply varies with the modulated signal;
 16. The terminaldevice according to claim 15, wherein the modulation circuit comprises:a conversion circuit, connected to the device ground of the terminaldevice, and the output and the sensor ground of the sensor unit, andconfigured to, upon modulating the driving signal to the modulatedsignal, output the modulated signal to the sensor ground; an energystorage capacitor, connected between the power supply and the sensorground, and configured to stabilize an operating voltage of the sensorunit; a high-speed transistor switch, connected to the power supply, andconfigured to perform synchronous switch-on and switch-off according tostatus of the conversion circuit, such that a voltage of the powersupply varies with the modulated signal; and a power source, connectedto the conversion circuit and the power supply of the sensor unit viathe high-speed transistor switch to supply power for the conversioncircuit and the sensor unit.
 17. The terminal device according to claim16, wherein the sensor unit is connected to a main control module of theterminal device via a communication port, and configured to output adriving signal having a low level between modulation idle intervals,such that a level of the device ground is approximately equal to a levelof the sensor ground.
 18. The terminal device according to claim 17,wherein: the sensor unit is directly connected to the main controlmodule via a conducting wire, and the communication port is maintainedat a low level while the conversion circuit modulating the drivingsignal; or the fingerprint identification sensor further comprises aresistor array, the sensor unit being connected to the main controlmodule via the resistor array; or the fingerprint identification sensorfurther comprises a relay module, the sensor unit being connected to themain control module of the terminal device via the relay module.
 19. Theterminal device according to claim 18, wherein when the sensor unit isconnected to the main control module via the relay module, the relaymodule receives data sent by the sensor unit and caches the data, andthe main control module acquires the data from the relay module.
 20. Theterminal device according to claim 19, wherein the sensor unit and therelay module are integrated in a sensor chip.